1. Field of the Invention
The field of the invention relates to mammography.
2. Description of Related Art
In the area of application of an embodiment of the invention, one or more X-rays of a breast are taken, e.g. to investigate the breast tissue with a view to detecting the presence of tumours for example.
For this purpose, a device is used to compress the breast to be X-rayed between two compression members, so as to position the breast at the point provided for image acquisition and to obtain a substantially constant thickness of the breast between the two compression members over a certain width thereof, to achieve an image of the breast having substantially homogenous contrast over this width.
During a mammography examination requiring a series of images to be taken, the patient's breast remains between the compression members for ten seconds to a few minutes.
Said examinations using a series of images are described for example in the references given below: [1] concerning digital breast tomosynthesis (DBT), [2] concerning contrast-enhanced digital breast tomosynthesis and [3] concerning temporal contrast-enhanced digital mammography (Temporal CEDM).
In general, the objective of these methods is to take a limited number of breast X-ray images and, using adequate image processing systems, to reconstruct a three-dimensional, digital volume of the breast. The reconstructed three-dimensional volume consists of a plurality of reconstructed two-dimensional images called sectional planes, parallel to the plane of the X-ray detector and stacked along the axis perpendicular to the detector plane.
The compression exerted by the compression members on the breast does not, however, prevent the breast from moving as and when the successive X-ray images are taken.
Breast movements during image acquisition create artefacts and localization errors on the images, which may lead to deterioration of image quality and even make images unusable.
Frequently, several successive images of the breast are therefore taken at different angles using the same image acquisition apparatus.
If the acquisition time is short (a few seconds) the female patient can be requested not to breathe and not to move.
If the examination lasts longer, breathing and involuntary movements are difficult to prevent.
Movement of the breast, when an image is being taken, blurs the image. If the breast has moved between two images, there is no blur but shifting of the breast between the two images which may give rise to problems during post-processing of the images, thereby distorting the three-dimensional reconstruction of the breast.
Document U.S. Pat. No. 7,187,749 describes a blur compensation device for X-ray images, in which a sensor is moved by the movement of the object to be imaged, in this case a person's head. The sensor detects the change in position relative to a reference position on the basis of acceleration, angular speed or the angle of a supporting means for the object. Compensation means are provided to compensate for blur on a plurality of images, on the basis of movement calculated using the change in position, and the series of images thus compensated is then stored.
Document U.S. Pat. No. 5,099,503 describes a device to control the functioning of mammography equipment, in which the compression force applied by the breast holding means is measured. On the basis of this measurement, the speed of the compression movement is adjusted. When the breast is compressed by a compression plate, the speed of the compression motor is slowed when the compression force increases and, when the maximum allowable compression force has been reached, the compression motor is completely stopped to avoid excessive compression forces.
Document U.S. Pat. No. 5,023,894 relates to a device to correct data in a topographic image of a patient's head, in which head movement detection means, comprising a marker affixed to the head and a detector sensitive to the position of the marker or a camera, are provided to detect movements of the head. The head movement sensor is coupled to an address converting unit, which converts the data detected by the sensor into address correction data, in order to correct the tomographic image of the head.
Document US-A-2007/0025503 describes a method to acquire mammographic images which provides for image data acquisition by an X-ray system and automatic analysis of the acquired images by said system, analysing blur due to movement, and indicates whether there is blur due to movement.
These known prior art systems have several drawbacks.
The compensation or correction of an image by calculation, in the event of breast movement, is cumbersome and complicated to apply. In practice it is difficult to implement automatic image or image data analysis that is satisfactory. Additionally, image correction is most often oversized compared with the actual needs of a mammography technician.
A marker placed on the breast is not recommended for the following reasons:    the marker will inevitably lie in the field of the images and may affect the images taken if it is radioopaque,    the positioning of the marker must be fairly precise and constant irrespective of breast morphology, so that it remains within the detection limits of movement,    the movement system must be initialized so that it records the initial position of the marker after breast compression,    the examination is already sufficiently traumatizing and painful for the patient without adding any additional steps to the imaging protocol.
Finally, control over the speed of the motor moving the breast compression plate is sufficient to achieve proper breast compression before taking any images, but does not prevent the breast from moving once the motor is stopped and the compression plate has been immobilized.